1. Field of the Invention
This invention relates to amplifier circuits and in particular to amplifier circuits which include diode compensation circuits to improve the amplifier characteristics by offsetting varactor error currents within the amplifier circuit.
2. Description of the Related Art
In amplifiers, the collector-base capacitances (C.sub.BC) of certain component transistors are connected in parallel with compensation devices and, consequently, effect the frequency response of the amplifier. To the extent these C.sub.BC s are known, their impact can be reduced by adjusting the size of the compensation device accordingly. However, the C.sub.BC of a transistor varies with voltage changes across the collector-base junction as occur, for example, when the voltage at the base or collector of the transistor is driven by a time-dependent input signal. In this case, the collector current of the transistor includes a varactor error current due to variations in C.sub.BC with voltage, in addition to the current component generated by true transistor action.
For example, the current into a node that includes a voltage dependent capacitance is related to the voltage change at the node as follows: EQU (Eq. 1)I=d(CV)/dt+V(dC/dV)dV/dt.
Here, V (dC/dV) dV/dt represents the varactor error current, which is a function of the node voltage, V. For example, dC/dV varies as V.sup.-3/2 for a step graded junction, producing a voltage dependent error current that varies as V.sup.-1/2.
Varactor error currents are significant where the current represented by Eq. 1 is coupled to the gain node of the amplifier circuit. For example, varactor error currents cause the bandwidth and stability of an amplifier to change with the supply voltage, and processing variations in the C.sub.BC s of transistors contribute errors that cannot be offset by conventional compensation schemes. Further, the bandwidth and stability of an amplifier will vary with signal voltages, leading to high frequency distortion, differential gain errors, and differential phase errors. Because of their dynamic nature, these error currents are difficult to offset.
Attempts have been made to offset the impact of varactor error currents on the performance of amplifier circuits. For example, in the neutralization method the average value of a varactor error current at a gain node is estimated, and an inverted version of this average varactor error current is generated and fed back into the gain node to offset the actual varactor error current. Because the correction signal is only a steady state approximation and does not reflect the dynamic voltage dependence of the varactor error currents, only partial compensation is achieved.